Microporous flat sheet and hollow fiber membranes are well known in the art. See, for example, U.S. Pat. Nos. 4,230,463 and 4,772,391. Such membranes are typically made by a solution-casting process (flat sheets) or by a solution precipitation process (hollow fibers), wherein the polymer is precipitated from a polymer/solvent solution. Conventional polymers used for microporous membrane formation by solution precipitation are not resistant to the solvents used to form the polymer solution for the casting or spinning fabrication, or to solvents of similar strength since such solvents dissolve or swell the polymer. Thus, membranes made from conventional polymers cannot be used to treat feed streams containing solvents or other harsh chemicals.
The manufacture of solvent-resistant membranes from polyimides is well known in the art. See, for example, commonly assigned U.S. Pat. No. 5,753,008. This patent discloses a process for spinning a fiber from a precursor polymer, and then rendering the fiber solvent-resistant in a post-casting step. Such membranes are indeed solvent-resistant. However, polyimides are known to be susceptible to hydrolysis when exposed to water at elevated temperatures. As a result, these solvent-resistant microporous polyimide fibers are not suitable for applications where the stream to be treated is hot and contains water.
One polymer that has been shown to be stable to hot water is polybenzimidazole (PBI). PBI has also been shown to be chemically resistant after crosslinking. See, for example, U.S. Pat. Nos. 4,693,824, 4,020,142, 3,720,607, 3,737,042, 3,841,492, 3,441,640, 4,693,825, 4,512,894 and 4,448,687. In these patents, various processes for making membranes from PBI are disclosed. However, the resulting membranes are not microporous, but instead have a dense skin on at least one surface, resulting in low permeation rates. These patents also disclose a number of techniques for crosslinking the PBI membranes. However, these crosslinking procedures lead to a dramatic increase in the brittleness of the membrane, making them difficult to manufacture and use.
There are several aspects of the present invention.
In a first aspect, the invention comprises a microporous solvent-resistant hollow fiber membrane formed from polybenzimidazole (PBI), the membrane being characterized by exceptional nitrogen permeance, high tensile strength and high elongation at break, making it particularly well-suited as a coatable support for fabricating composite permselective membranes.
In a second aspect, the invention comprises a method of making such a solvent-resistant PBI membrane.
In a third aspect, the invention comprises a countercurrent flow separation module incorporating a composite membrane wherein at least one selective coating is placed on a surface of such a solvent-resistant PBS membrane.
In a fourth aspect, the invention comprises a method of crosslinking a membrane (hollow fiber, flat sheet, or tubular; microporous, isoporous, non-porous, or asymmetric) formed from PBI using a multi-functional alkyl halide.
The membranes of the present invention are useful for a variety of applications, including ultrafiltration, microfiltration and membrane contactors; and as supports for composite membranes that are used in such applications as reverse osmosis, nanofiltration, pervaporation, vapor permeation and gas separations.
In contrast to the procedures of the prior art, it has now been found that microporous PBI membranes with exceptional performance and solvent resistance, can be made by proper selection of the procedures for making and crosslinking the membranes.
In one aspect, the invention comprises a microporous hollow fiber membrane formed from PBI, the membrane being fabricated by the following steps:
(a) providing a fiber-spinning polymer solution comprising 15 to 30 wt % PBI, 2 to 5 wt % high molecular weight pore-former with a molecular weight of  greater than 1000 daltons, 5 to 30 wt % low molecular weight pore-former, with a molecular weight of  less than 100 daltons, and a solvent;
(b) forming a spun membrane by extruding the polymer solution through an orifice at a temperature of 25xc2x0 to 60xc2x0 C. while simultaneously injecting a coagulating fluid through a needle located in the orifice;
(c) providing a quench bath;
(d) passing the spun membrane through the quench bath at a temperature of 10xc2x0 to 40xc2x0 C. to form a microporous hollow fiber membrane; and
(e) rinsing the membrane.
Additional optional steps include drying and post-treating the membrane by crosslinking or annealing.
The microporous hollow fiber PBI membranes formed by this process have excellent properties for a wide variety of membrane separation processes. Generally, the membranes have a gas permeance of at least 5 m3/m2xc2x7hrxc2x7atm, preferably at least 10 m3/m2xc2x7hrxc2x7atm. In addition, the surface pores on the membrane (both inside and outside surfaces of the hollow fiber) are greater than about 0.05 xcexcm, and less than about 1 xcexcm. The fibers have a tensile strength of at least 100 g/filament, preferably at least 200 g/filament. The fibers also have an elongation at break of at least 10%, preferably at least 15%. The fibers can also be made with a wide range of diameters and wall thicknesses, depending on the application of use. Generally, the inside diameter of the fibers can range from about 200 xcexcm to about 1000 xcexcm, and the wall thickness of the fibers can range from about 30 xcexcm to about 200 xcexcm.
The invention can be used with virtually any PBI, and in particular with those described in U.S. Pat. Nos. 2,895,948, 5,410,012, and 5,554,715, the disclosures of which are incorporated herein by reference. These PBIs have the following general repeat structure: 
where R is a tetravalent aromatic nucleus, typically symmetrically substituted, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms of the aromatic nucleus, and Rxe2x80x2 is selected from (1) an aromatic ring, (2) an arylene group, (3) an alkylene group, (4) an arylene-ether group, and (5) a heterocyclic ring, such as a pyridine, pyrazine, furan, quinoline, thiophene, or pyran. A preferred PBI is poly(2,2xe2x80x2-[m-phenylene])-5,5xe2x80x2-bis-benzimidazole.
It has been found that to obtain a microporous fiber with high porosity and high gas permeance while maintaining excellent physical properties such as high tensile strength and elongation at break, a mixture of a high molecular weight pore-former and a low molecular weight pore-former should be used. The weight ratio of high molecular weight pore-former to low molecular weight pore-former should range from 0.05 to 0.5, preferably from 0.075 to 0.25.
The high molecular weight pore former should have a molecular weight of at least about 1000 daltons. It should also be soluble in the solvent used for the fiber-spinning polymer solution and in the materials used for the internal coagulation solution and the quench bath. Examples of suitable high molecular weight pore-formers include polyvinyl pyrollidinone (PVP), polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), polyethylene glycol (PEG), and polypropylene glycol (PPG). A preferred high molecular weight pore-former is PVP.
The low molecular weight pore-former should have a molecular weight of no greater than about 100 daltons, and should be hydrophilic. It should also be soluble in the solvent used for the fiber-spinning polymer solution and in the materials used for the internal coagulation solution and the quench bath. In general, the class of useful low molecular weight pore-formers comprises (i) a lower alkanol, (ii) a polyfunctional alcohol, (iii) ester and ether derivatives of an alkanol, (iv) ester and ether derivatives of a polyfunctional alcohol, (v) mixtures of (i)-(iv), and (vi) mixtures of water and at least one of (i)-(v).
Examples of suitable low molecular weight pore-formers include monofunctional alcohols, such as methanol (MeOH), ethanol (EtOH), isopropyl alcohol (IPA), n-propanol, and the various isomers of butanol; polyfunctional alcohols, such as ethylene glycol, propylene glycol, and glycerol; and ether and ester derivatives of monofunctional and polyfunctional alcohols. A preferred low molecular weight pore-former is n-propanol.
Preferred solvents for the fiber-spinning solution include dimethylacetamide (DMAc), dimethylformamide (DMF) and N-methyl pyrrolidone (NMP). The fiber-spinning polymer solution preferably is filtered to remove oversize particles and lumps through a fine gage (10-30 xcexcm) filter, and has a viscosity of from 15,000 to 50,000 cp at the spinning temperature, which is preferably conducted at from 250 to 60xc2x0 C. Fiber-spinning or extrusion is conducted at an extension rate of from 1 to 5 cm3/min, depending upon the spinning solution viscosity and the temperature at which the extrusion is conducted. A preferred extrusion rate is 2 cm3/min. Conventional tube-in-orifice spinnerets may be used, typically having an orifice diameter on the order of 500 to 1500 xcexcm and a tube on the order of 25- to 30-gage.
Both the internal coagulation solution and the quench bath preferably comprise a polar solvent selected from MeOH, EtOH, n-propanol, IPA, DMAc, water and mixtures thereof. Rinsing is preferably conducted with water and/or IPA.
In another aspect, the invention comprises a method for crosslinking a PBI membrane by the following steps:
(a) providing a crosslinking solution comprising a multi-functional alkyl halide in a solvent;
(b) soaking the membrane in the crosslinking solution for 0.5 to 48 hours and at a temperature from 50 to 150xc2x0 C.; and
(c) drying the membrane for 0.5 to 48 hours at a temperature of 25xc2x0 to 200xc2x0 C.
The multi-functional alkyl halide should contain at least two halide substituents, and has the general structure 
where X is Br or Cl, n is 1 to 11, a is 1 to 10, b is 0 to 4, and c is 0 to 6. A preferred class of difunctional alkyl halides comprises straight chain, terminally di-substituted compounds having the structure
Xxe2x80x94(CH2)nxe2x80x94CH2xe2x80x94X
where X and n are as defined above. A most preferred difunctional alkyl halide is dibromobutane (DBB).
The alkyl halide may also contain three or more halide substituents. Exemplary alkyl halides with three or more halide substituents include tribromopropahe, trichloropropane, pentaerythrityl tetrabromide, and pentaerythrityl tetrachloride.
The solvent used to dissolve the alkyl halide should not react with the alkyl halide and should not dissolve the uncrosslinked PBI membrane. Preferred solvents include ketones, such as acetone, methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), and pentanone; and ethers, such as isopropyl ether and butyl ether. The resulting crosslinked PBI membrane has exceptional chemical and thermal resistance.
In another aspect, the invention comprises a crosslinked microporous hollow fiber membrane formed from PBI, the membrane being fabricated by the following steps:
(1) providing a fiber-spinning solution having the makeup noted above;
(2) forming a spun membrane by extrusion as noted above;
(3) passing the spun membrane through a quench bath as noted above to form a microporous hollow fiber membrane;
(4) rinsing the membrane; and
(5) crosslinking the membrane as noted above.
In another aspect, the invention comprises a composite hollow fiber membrane comprising at least one permselective coating formed on a crosslinked microporous PBI hollow fiber membrane made as described above. The permselective coating that is applied depends upon the particular separation it is desired to achieve, such as the removal of water vapor from organics, the removal of volatile compounds from water vapor, the separation of organics or the purificationn of water.
In yet another aspect of the invention there is provided a countercurrent flow separation module comprising:
(a) a chamber having feed and retentate ends and means for removing permeate vapor near the feed end;
(b) a bundle of thin film composite hollow fiber membranes arranged substantially parallel to each other in said chamber, each of said composite hollow fiber membranes comprising a solvent-resistant PBI hollow support fiber having at least one permselective coating on a surface thereof, the PBI support fiber having been formed by and optionally crosslinked by the methods noted above; and
(c) means for securing and sealing the bundle of hollow fiber membranes to the chamber at its feed and retentate ends so as to permit fluid communication with a feed stream.
Details of the construction and operation of such a vapor separation module are exemplified in Examples 28-31 herein and in commonly assigned U.S. Pat. No. 5,753,008, the pertinent disclosures of which are incorporated herein by reference.
For the removal of water from a feed stream, it is best that the permselective coating material be more permeable to water than to other components in the feed stream. In this case, the material is preferably very hydrophilic. Examples of permselective coating materials useful for removing water from organics include polyvinyl alcohol (PVA), cellulosic materials, chitin and derivatives thereof, polyurethanes, polyamides, polyamines, poly(acrylic acids), poly(acrylates), poly(vinyl acetates), and polyethers. Other polymers normally viewed as not especially hydrophilic such as polyolefins, polystyrene, and poly-acrylates can be rendered sufficiently hydrophilic to be selective to water vapor by incorporating hydrophilic groups such as hydroxyl, amine, carboxyl, ether, sulfonate, quaternary amine, carboxyl, ether, sulfonate, phosphonate, quaternary amine, and ester functionalities. Such groups can be incorporated by choosing monomers that contain such groups or by adding them in a post-treatment step such as radiation- or plasma-grafting. Blends and copolymer versions of these materials are also useful. The coating material should also be crosslinked to provide sufficient resistance to swelling or dissolution by components of the feed stream.
A particularly preferred permselective coating material for dehydration of organics is a blend of PVA and polyethyleneimine (PEI), wherein the material is crosslinked through the amine groups of the PEI using ethyl succinate by heating to elevated temperatures. By varying the ratio of PVA to PEI or the amount of ethyl succinate crosslinking agent used, the selectivity and permeability of the membrane may be adjusted. This coating will be extremely effective for vapor permeation applications. However, it will also prove useful for other separations including dehydration of organics by pervaporation; the removal of water vapor from compressed gas streams, such as air and natural gas; and for use in fuel cells, allowing the transport of water while restricting the passage of hydrogen.
A particularly preferred class of permselective coating materials for water purification by reverse osmosis or nanofiltration is polyamides formed by interfacial polymerization. Examples of such coatings as found in U.S. Pat. Nos. 5,582,725, 4,876,009, 4,853,122, 4,259,183, 4,529,646, 4,277,344 and 4,039,440, the pertinent disclosures of which are incorporated herein by reference.
For the removal of volatile compounds from water or gas streams such as air or nitrogen, the permselective coating is usually, but not always, an elastomeric or rubbery polymer. Examples of materials useful for such separations include natural rubber, nitrile rubber; polystyrene-butadiene copolymers; poly(butadiene acrylonitrile) rubber; polyurethanes; polyamides, polyacetylenes; poly(trimethylsilylpropyne); fluoroelastomers; poly(vinylchlorides); poly(phosphazenes), particularly those with organic substituents; halogenated polymers, such as poly(vinylidene fluoride) and poly(tetrafluoroethylene); and polysiloxanes, including silicone rubber. Blends and copolymer versions of these materials are also useful. Ion exchange membranes and composites may also be used for some applications. A particularly preferred coating for the removal of volatile compounds from water or gas streams is poly(dimethylsiloxane) and derivatives thereof.
For separation of organic mixtures, the choice of permselective coating material will depend on the organics being separated. Many of the polymers listed above for the dehydration of organics or the removal of volatile organics from water or gas streams will work well for separating certain organic mixtures. In particular, it is common to use copolymers for separating organics since the ratio of the so-called xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d segments can easily be adjusted to provide the desired selectivity.
The permselective coating material may be placed on the surface of the support fiber using a number of conventional techniques, including dip-coating, painting, spray-coating, solution-coating, or by interfacial polymerization. The coating may be placed on the inside (lumens) or outside surface of the support fiber; in most applications it is preferred that the coating be placed on the lumens.